The invention relates to a circular polarizer with voltage-controlled color selection. The polarizer comprises a liquid crystal cell with two carrier plates which are provided on their mutually facing sides with electrically conductive coatings (electrodes), and which enclose between them a chiral, color-selectively reflecting liquid crystal layer. Such an arrangement is known from Mol. Cryst. Liq. Cryst. Lett. 64 (1980) 69.
When their molecules are in a planar orientation, cholesteric liquid crystals show a characteristic optical effect, namely that they reflect light within a narrow frequency band which usually spans only a few nanometers. The reflected light is circularly polarized, namely in the same rotational sense as that in which the liquid crystal molecules are twisted relative to one another. The reflection maximum lies at a wavelength .lambda..sub.max which, with perpendicular incidence of the light, corresponds to the pitch of the liquid crystal helix and decreases with increasing angle of incidence. The transmission behavior of these liquid crystals is complementary, that is to say, in a spectral region centered around .lambda..sub.max, the only light which is allowed to pass through is that which is circularly polarized in the sense opposite to that of the reflected radiation. Light of other colors passes through unaffected.
The wavelength of the reflection maximum depends not only on the direction of observation, but reacts also to temperature changes and, above all, can also be varied by applying an electric voltage, as has been known for a long time (IEEE Trans.on Electron Devices ED-15 (1968) 896). This electro-optical effect, in itself, makes a number of interesting applications possible, for example in the field of metrology or, for instance, in the recording and reproduction of images, but it has hitherto been unable to find acceptance in practice. The main reason is that the reflection band can in principle be shifted only within relatively narrow limits; thus, the literature reference quoted above reports .lambda..sub.max shifts which, in spite of careful selection of the liquid crystal, do not exceed 30 nm. Moreover, it is also rather difficult to give the liquid crystal a planar texture which remains table and free from disturbances even under the action of a field.
The choice of colors becomes wider when a cell with a nematic liquid crystal between crossed linear polarizers is used instead, and the birefringence properties of the liquid crystal are changed by means of the electric voltage (Techn. Mitt. AEG-Telefunken 62 (1972)3). Even here, however, the liquid crystal molecules must be uniformly preoriented. A further difficulty is that useful results are obtained only if the liquid crystal layer is illuminated with a bundle of parallel rays and has a thickness within an extremely narrow tolerance.
Starting from this state of the art, it is the object of the invention to indicate a color-selective circular polarizer based on liquid crystals, which can be modulated within relatively wide limits and, in addition, can be produced without excessive costs. To achieve this object, it is proposed, according to the invention, that in an arrangement of the type described above, the chiral liquid crystal layer be converted to an optically isotropic phase.
This phase, for which the term "blue phase" (BP) has become accepted in the meantime, has been intensively investigated in recent years. The phenomena observed hitherto are extraordinarily complex, and some of them are difficult to interpret, so that the conditions under which a BP is formed, and the way in which its molecules are ordered, are in reality not yet fully known. It is not even certain whether the blue phase is an independent phase in the strict sense, or whether merely a special texture of the cholesteric phase is involved. However, disregarding this question which has not yet been elucidated, the BP can be unambiguously identified.
The BP occurs when certain chiral systems are warmed, starting from the cholesteric phase, or are cooled, starting from the isotropic phase. It is stable within a narrow temperature range, which is at most a few degrees Centigrade wide, below the clear point. The BP is at most weakly birefringent or not at all; on the statistical mean, its molecules are distributed isotropically. There are at least two distinguishable, three-dimensionally ordered BP modifications, of which one (BPI) predominates at lower temperatures, while the other (BPII) predominates at higher temperatures. During the cholesteric/BPI and BPI/BPII transitions--both being probably first-order transitions--the entropy changes only very slightly; by contrast, the entropy difference during the BPII/isotropic transition is large. Both types of blue phase show the selective reflection which is familiar from the cholesteric phase, the reflection maxima lying at different wavelengths, which in general are shifted towards the red relative to .lambda..sup.chol.sub.max ; .lambda..sup.BPI.sub.max &gt;.lambda..sup.BPII.sub.max &gt;.lambda..sup.chol.sub.max frequently applies. A detailed description of the blue phase is to be found in the monograph "Liquid Crystals of One- and Two-Dimensional Order" Springer Verlag, 1980, pages 161-175, edited by W. Helfrisch and G. Heppke.